1. Field of the Invention
The present invention relates to a method for inhibiting oxidation of a copper film on a ceramic body. More specifically, the present invention relates to a method for inhibiting oxidation of a copper film formed as an electrode or a conductive device on a ceramic electric circuit component including a ceramic body.
2. Description of the Prior Art
One example of a ceramic electric circuit component of interest to the present invention is a ceramic capacitor. The electrode of a ceramic capacitor was generally formed using silver of high conductivity. Such a silver electrode was formed by coating a silver paste on a ceramic body and by baking the same. However, with the recent rise of the cost of silver materials, the ratio of the cost of such silver electrode to the total cost of a ceramic capacitor has increased. Thus, employment of such a silver electrode became a cause of the rise of the total cost of ceramic capacitors.
In such situation, an attention was given to development of an inexpensive electrode. As one approach, various studies were made as to a method for forming a metallic film by various processes, such as electroless plating process, vacuum evaporation process, sputtering process, ion plating process, and the like. As another approach, study was made as to employment of an inexpensive metal in substitution for the silver as an electrode.
The first approach employed was a nickel plated electrode formed by means of an electroless plating process. A nickel plated electrode was successful to some extent as an inexpensive electrode in substitution for a silver electrode. However, it was observed that employment of a nickel plated electrode as an electrode of ceramic capacitors involves the following problems. More specifically, the resistivity of a nickel electrode per se is 7.24.times.10.sup.-6 .OMEGA..cm, which is higher than that of silver which is 1.62.times.10.sup.-6 .OMEGA..cm. Accordingly, the frequency characteristic is degraded in the high frequency region. Another problem is that solderability of a nickel plated electrode is poor. Furthermore, another approach attempted that the whole surface is coated with a solder layer in order to decrease the resistivity of the nickel plated electrode. However, in coating the whole surface of the electrode with a solder layer, a large amount of flux which is active must be used. Accordingly, it is necessary to cleanse the electrode to remove unnecessary flux after soldering. Furthermore, although soldering the electrode portion is by dipping the same in a solder tub, for example, such process cause stress in the ceramics, so that a crack is liable to be caused in the ceramics.
Development of an inexpensive and high conductive electrode which can be substituted for the above described nickel electrode was clearly desired. Under the circumstances, a copper plated electrode made by means of a copper electroless plating process was attemped. However, it was observed that a serious obstacle is involved in a copper electroless plated electrode. More specifically, a copper plated electrode formed by an electroless plating process as such was liable to be oxidized and had a large resistivity as compared with that of a bulk of copper. A similar phenomenon was also observed in a copper electrode formed by a vacuum evaporation process, a sputtering process, an ion plating process and the like as well as a copper electrode formed by an electroless plating process.
The above described property of greater liability to be oxidized results in disadvantages. For example, when a copper film is used as an electrode of a ceramic capacitor, an unfavorable phenomenon is observed in which the conductivity of the electrode is decreased due to formation of an oxide film as a result of oxidation, which accordingly decreases solderability.
With regard to the fact that a copper film formed by the above described various processes has a large resistivity as compared with that of a bulk of copper, the copper film requires some additional treatment to exhibit a characteristic of a copper bulk by achieving metallization, dense formation, enhancement of adhesiveness and stabilization. Usually, heat treating is employed to that end. Generally, heat treatment is performed in an inert atmosphere so that the copper film is not reacted with oxygen. By subjecting the copper electrode to a heat treating process, a copper film formed by an electroless plating process, a vacuum evaporation process, a sputtering process, an ion plating process, and the like becomes for the first time a copper film exhibiting an electric characteristic close to that of pure copper and accordingly, a ceramic electric circuit component such as a ceramic capacitor of high reliability can be realized.
However, by subjecting a copper film to the heat treating process, the copper film comes to exhibit a property of greater liability to be oxidized as compared with a copper film not heat treated and of greater liability to a time dependent characteristic change. This is presumably due to the fact that a catalytic activity is provided to the copper film surface, thereby making it more liable to be oxidized.
Accordingly, a copper film is more or less liable to be oxidized irrespective of heat treatment and in any case, some oxidization inhibiting process is required to a copper film. Furthermore, a copper film after heat treatment particularly requires an oxidization inhibiting process.